Drawing and heat treatments of polyester filaments

ABSTRACT

PROCESS FOR DRAWING AND HEAT-TREATING POLYESTER FILAMENTS COMPRISING SUBJECTING UNDRAWN POLYESTER FILAMENTS TO A FIRST DRAWING AT A RELATIVELY LOW TEMPERATURE (T1), SUBJECTING THE FILAMENTS TO A SECOND DRAWING AT A TEMPERATURE (T2) LOWER THAN T1 AT DRAW RATIO AT LEAST 80* OF THE MAXIMUM DRAW RATIO AT THIS STAGE, SUBJECTING THE FILAMENTS TO A FIRST HEAT TREATMENT UNDER TENSION AT A TEMPERATURE (T3) HIGHER THAN T2, AND THEREAFTER SUBJECTING THE FILAMENTS TO SINGLE OR MULTI-STAGE SECOND HEAT TREATMENT PERFORMED AT TEMPERATURES HIGH THAN TEMPERATURE T3 UNDER TENSION, THE TEMPERATURE OF AT LEAST ONE STAGE OF THE SECOND HEAT-TREATMENT BEING 110*C.

DRAWING AND HEAT TREATMENTS OF POLYESTER FILAMENTS Filed Feb. 11, 1969 2 Sheets-Sheet 1 LOAD 7) 5 ELONG-ATION March 1972 YUKIO MITSUISHI ETAL 3,651,198

DRAWING AND HEAT TREATMENTS OF POLYESTER FILAMENTS Filed Feb. 11, 1969 2 ShQQtS-Sheet 2 2b ELONGATION (X) United. States Patent Int. Cl. D02j 1/22 Us. or. 264-235 v Claims ABSTRACT OF THE DISCLOSURE Process for drawing and heat-treating polyester filaments comprising subjecting undrawn polyester filaments to a first drawing at a relatively low temperature (T subjecting the filaments to a second drawing at a temperature (T lower than T at draw ratio at least 80% of the maximum draw ratio at this stage, subjecting the filaments to a first heat treatment under tension at a temperature (T higher than T and thereafter subjecting the filaments to single or multi-staged second heat treatment performed at temperatures higher than temperature T under tension, the temperature of at least one stage of the second heat-treatment being 110 C.

This invention relates to a novel process for drawing and heat-treating polyester filaments.

Polyester filaments are known for their high crystallinity and high melting point, and also for their other excellent properties such as high resistances to heat, chemicals and light, great strength and high elastic modulus. Because of those favorable properties, the filaments are very important as clothing and industrial materials. Particular- 1y, their importance as a tire cord material is rapidly increasing as a recent trend. While there are a number of requirements particularly important for tire cord material, the most significant are that the material should possess high strength, elongation, toughness, dimensional stability and fatigue resistance.

When polyester filaments as tire cord material are compared with polyamide filaments such as 6-nylon, 6,6-nylon, etc., the former excel in thermal dimensional stability, but are not necessarily superior to the latter in strength, elongation, and mechanical fatigue resistance. Therefore, it is expected that, if polyester filaments could retain the high dimensional stability and furthermore possess still improved mechanical strength, elongation, and fatigue resistance, such filaments will have much increased utilities not only as tire cord, but also in the field of clothing and interior decoration.

Various attempts have been made to improve such properties of polyester filaments. For example, the degree of polymerization of the polymer composing the filaments was raised in attempts to produce the filaments having high strength,.elongation and fatigue resistance. However, the degree of polymerization naturally cannot be raised beyond certain limitations incurred by operational factors,

and the attempts have not achieved satisfactory results..

We have engaged in extensive research over a prolonged period, for the purpose of making improved polyester filaments, and now completed the present invention.

An object of the invention is to provide a novel drawing and heat-treating process for making polyester filaments having high breaking strength, breaking elongation and large Youngs modulus.

Another object of the invention is to provide a drawing and heat-treating process consisting of unique and novel combination of drawing steps and heat treatments, for

ice

making polyester filaments particularly suited as tire cord having high toughness, fatigue resistance, and a strengthconversion ratio (percent conversion of the strength of the starting yarn).

According to the invention, a drawing and heat-treating process of polyester undrawn filaments containing at least mol percent of ethylene terephthalate units is provided, which comprises the following steps to be performed in the order stated:

1) First stage drawing wherein the undrawn filaments are drawn by l.1-4.0' at a temperature within a range of 40100 C.

(2) Second stage drawing wherein the drawn filaments are further drawn at a temperature within a range of 10-85 C. and lower than that employed in the step (1) at a draw ratio within a range of 1.2-7.0x and which is at least 80% of the maximum draw ratio,

(3) First heat treatment wherein the filaments are heattreated at a temperature within a range of 55 -l20 C. and higher than that employed in the step (2) while being subjected to a tension that will maintain the filament length during the heat-treatment at 130% of that immediately before said treatment, and

(4) Second heat treatment wherein the filaments are heat-treated, at least once, at temperatures higher than that employed in the step (3), at least one stage of the heat treatment being performed at a temperature not lower than C., the filaments being subjected in each stag to a tension that will maintain the filament length treatment at 9 5-130% of that of an immediately preceding stage, and furthermore the tension exerted on the filaments in each stage of step (4) being so adjusted that the filament length after the final stage of step (4) becomes at least 90% of that immediately before step (3).

The characteristic features of the invention in sequence, are as follows: preparation of highly oriented but non-crystalline drawn filaments of polyester by twostage drawing of polyester undrawn filaments at relatively low temperatures as described in the steps (1) and (2): preparation of drawn filaments of polyester having a uniform structure in the directions both parallel and perpendicular to the filament axis, and being highly oriented but still non-crystalline from the drawn filaments obtained in steps (1) and (2), by the first heat treatment under tension of step (3); and preparation of polyester filaments having a very uniform structure in the directions both parallel and perpendicular to the filament axis and being highly oriented and highly crystalline from the drawn filaments obtained in step (2) by the singleor multi-stage second heat treatment of step (4).

The polyester filaments thus obtained in accordance with the invention exhibit remarkably high breaking strength and breaking elongation due to their unique structure, and as the result possess very high toughness. When used as tire cord, they show excellent fatigue resistance and high strength-conversion ratio. Furthermore, since they have very high Youngs modulus, the drawback of fiat spots is remarkably improved, when compared with nylon tire cord. Thus the polyester filaments obtained in accordance with the subject process are clearly distinguishable from conventional products, in that they are simultaneously excellent in all of the many important mechanical properties.

The critical feature of the process of this invention resides in the combination of steps (1) through (4) as aforesaid. As will be demonstrated in the later gi-ven examples and controls, even when only one condition out of the many operational conditions specified as to the four steps is outside the specified range, the resulting drawn filaments of polyester lose the advantageous char: acteristics of the polyester filaments of this invention,

3 i.e., the simultaneous superiority in many mechanical properties.

FIG. 1 is a load-elongation curve of the polyester filaments prepared in accordance with one embodiment of the process of this invention. FIG. 2 shows load-elongation curves of the polyester filaments prepared in accordance with another embodiment of the process of this invention and the polyester filaments prepared in accordance with the conventional process.

Hereinafter the process of the present invention will be explained in further detail.

The polyester undrawn filaments employed in the invention are composed of high molecular weight polyethylene terephthalate, i.e., a polymer containing at least 85 mol percent, preferably at least 90 mol percent, of a recurring structural unit of the formula,

The undrawn filaments may be those which are spun by optional spinning means conventionally employed.

Thus the term polyester is used in the specification and claims, in the sense including modified polyethylene tcrephthalate by the addition of no more than approximately 15 mol percent of other ester-forming units. As such minor, other ester-forming units, the following may be named'by Way of examples: diethylene glycol, other polymethylene glycols of 1-10 carbons, hexahydro-pxylylene glycol; aromatic dicarboxylic acids such as isophthalic, dibenzoic, p-terphenyl-4, "-dicarboxylic, and hexahydroterephthalic acids; aliphatic acids such as adipic acid; hydroxy acid such as hydroxyacetic acids; and the like.

The polyester filaments employed in the invention are inclusive of monofilaments.

The properties of the undrawn filaments are not critical, but the filaments having an intrinsic viscosity of at least 0.3 and a birefringence ranging 0.0005-0.0120 are preferred. The undrawn filaments having a birefringence outside the specified range tend to produce drawing failure or increase filament breakage during drawing operation. The density of the undrawn filaments is preferably no higher than 1.35 g./cm. Otherwise the filament breakage during drawing tends to be increased.

The values denoting the properties of polymer and filaments given in the specification are measured as follows.

Intrinsic viscosity of a polymer is given as a norm of degree of polymerization of that polymer, which is defined below:

In the formula, v is normally referred to as relative viscosity, obtained by dividing the viscosity of a dilute solution of a polymer by the viscosity of employed solvent which is measured at the same temperature. Also is the polymer concentration in the solution expressed by g./100 cc. The intrinsic viscosities given in the present specification are calculated from the values measured at 35 C., using ortho-chlorophenol as the solvent.

It is well known that load-elongation curve and breaking strength and breaking elongation calculated from the curve are variable in shape and value according to the length of test specimen and extension rate. In the present specification, the tensile. test is performed with 204cm.

long samples at an extension rate of"50%/min. under standard conditions (20 C., relative humidity of 65%), using an Instron tensile tester.

Also the grade of inclination on load-elongation curve is expressed by differential coefiicient of strength tovthe elongation at each point on the load-elongation curve, viz., grade of tangent. Incidentally, correction-of the values with respect to the decrease infilament deniers accompanying the elongation is not performed.

According to the subject processfthe polyester undrawn filaments are first subjected to the first stage drawing, to be drawn by 1.1-4-0X. Preferably 1.2-3.0X, at a temperature within within a range of 40-100 0., preferably 60-100 C., as described in step 1). Optional heating medium may be employed in this step, i.e., non-solvent liquids being preferred. ,7

The filaments are then subjected to "the second stage drawing in the step (2), under the following conditions. That is, the filaments are drawn at a temperature within a range of 1085 (3., "preferably 4075 C., which is lower than the drawing temperature employed in the first stage (step (1)), preferably lower by at'leastj 10 C.,' r

with a draw ratio withina range of 1 .27.0 and which is at least of the maximum attainable draw ratio under the second stage drawing conditions, preferably that within a range of 1.55.0 and which is at least of the, aforesaid maximum draw ratio. The type of heating medium in this step is not critical, but non-solvent'liquids such as water and ethylene-glycol are preferred.

The above first and second stage drawings can be practiced with any optional known drawing machine conimonly employed.

The term, maximum draw ratio tion and claims signifies the maximum draw ratio of the filaments under the specific drawing conditions, at which no filament breakage takes place. Obviously the maximum draw ratio is dependent on the properties of the filaments to be drawn, but is also affected by the drawing conditions such as temperature, mediumv in which the filaments are drawn, and drawing rate, etc. I

The polyester filaments after' drawing are then subjected to the first heattreatment instep (3), under the following conditions. That is, the filaments are heat-.

treated at a temperature within a range of 55 -120 C.,

preferably 65 -100 Cj, and-which is higher than the tem-. perature employed in the second stage drawing, preferably higher by at least 10 v U During the heat treatment, the polyester filaments must be under a tension there will maintain the filament length at 130% of that immediately beforethe step 3), i.e., after completion of step (2 In other words, the polyester filaments are either stretched or elongated by at most 30%, or maintained at the same angel, shrunk by at most 5%.

The polyester filaments of the invention exhibit a marked tendency for shrinkage after the two-stage drawing of steps (1) and (2). as above-described, and under the temperature condition applied in the heat treatment of step (3), normally show a free shrinkage of at least 20% The term free shrinkage used herein means spontaneous shrinkage expressed by percent, which occurs when the filaments are exposed to the temperature'condition free from tension. Therefore, when the polyester filaments are maintained at the state'that will allow no more than 5% shrinkage during the heat treatment of step (3),

the filaments are given a sufiicient tension since the free. shrinkage is far greater than the allowable shrinkage. Consequently, the filaments never exhibit configurational change such as formation. of crimps,- and are free from i.

relaxation. r g g The term heat treatment under tension used in the specification and claims signifies a heat treatment which is performed while either restricting the filaments ata limited shrinkage of not more than 5 which is far less than the free shrinkage the same filaments under the employed heat-treating conditions, or maintaining their constant length, or extending the filaments with astretch of 'singleormulti-stage heat treatments under tension.

The treating temperatures in all of the stages under tensu ch' as water and ethylene-glycol used in the specificasion of step (4) must be higher than the temperature em; ployed in step (3 preferably byyat least lfl" (3., andvin at least one stage'of the heat treatment of step .(4), the filaments must be subjected to heat treatrnentat a temperatnre not lower .than 110 C., preferably not lower than 120 C., more preferablynot lower than 150 C.

.In accordance with'the subjectprocess, it is essential that at each stage of the h'eattreatments constituting the step (4), the filaments are maintained at such a state to have a length corresponding to 95-130% of that of the filaments after completion of the preceding heat treatment stage and immediately before the pertinent stage. Furthermore, the tension at each stage must be so adjusted that the filaments after the final heat treatment stage of step (4) should have a filament length corrmponding to at least 90% of that of the filaments after the second stage drawing of step (2) and immediately before the first heat treatment of step (3). 7

When the filaments are maintained at such a state in each stage of they step (4), heat treatment satisfying the above two requirements, the filaments are always tense throughout the second heat treatment, by the same reasons as described as to the first heat treatment.

The limited or allowable shrinkage in the heat treatments of steps (3) and'(4) is preferably not more than 3%, and generally more satisfactory results are obtained when the heat treatments are. performed maintaining constant filament length or under stretching by at most 30%, rather than the cases of heat-treating the filaments with a shrinkage within the specified range. The treating time of either step (3) or (4) is not critical, but normally that ranging 0.0ll,000 seconds is preferred. The heat treatment of step (3) and that of step (4) may be practiced using separate apparatus, or using a single heat-treating apparatus comprising multi-zones, in each of which the heat treatment of each stage isperformed respectively.

One characteristic feature of the subject process is that a total draw ratio (TDR) markedly higher than that ohtainable in conventional methods can be realized, due to the adoption of aforesaid unique combination of drawing and heat-treating steps. Namely, the filaments which completed the foregoing steps (1) through (4) can have a length 6 to times that of the undrawn filaments, and with the adoption of such extremely high total draw ratio, the advantagesof the subject process are best manifested.

Also the heat-treating temperature of step (4) can be raised as high as 600 C., or even higher, if, for example, a known non-contact type salt heater is employed.

As has been explained in the foregoing, the characteristers of the subject process reside in the combination of o the steps (1) through (4), while various modifications are permissible within the specified ranges of many operational conditions.

For example, polyester filaments obtained in one embodiment of the subject process can show a very characteristic type load-elongation curve.

It is generally known that a load-elongation curve of polyester filaments has first and second yield points. The yield point present at the side of low load and low elongation is referred to as the first yield point, and the other, as the second yield point. Whereas, in the aforesaid characteristic load-elongation curve, the presence of first yield point is not very clear, while the second yield point is quite distinct, as illustrated in FIG. 1. Also on the loadelongation curce, the grade of inclination after the second yield point is characteristically very small. In an optimum embodiment, polyester filaments showing very unique mechanical behavior quite different from that of conventional products, such as a minimum grade of inclination of no more than 9 g./d. above the point of 7.0 g./d. on the load-elongation curve, can be obtained in accordance with the invention.

As one embodiment of the subject process, if the step (4) comprises two heat treatments and if all of the heat treatments of steps (3) and (4) are performed while the filament length in every stage 95-ll0% of that immedi-.

ately before that stage, and furthermore if the tensions are so adjusted that the filaments after the second heat treatment ofstep (4) have a length corresponding to at least 90% but less than 120% of that of the filaments immediately before the heat treatment of step (3), the resulting filaments can show the above-described type of load-elongation curve.

More specifically, the polyester filaments showing this type of load-elongation curve can be prepared by an embodiment of the invention, i.e. a drawing and heat-treating process of polyester undrawn filaments containing at least mol percent, preferably at least mol percent, of polyethylene terephthalate units, which comprises the following steps to be performed in the order stated:

(1) First state drawing wherein the undrawn filaments are drawn by 1.1-4.0X, preferably by 1.23.0 in a non-solvent liquid of a temperature within a range of 40- C., preferably 60-l00 C.,

(2) Second stage drawing wherein the drawn filaments are drawn in a non-solvent liquid of a temperature within a range of 10-85 0, preferably 40-75 C., and which is lower than the temperature employed in the step (1), preferably by at least 10 C., at a draw ratio of 1.2-7.0 preferably 1.5-5.0X, and which is at least 80% of the maximum draw ratio,

(3) First heat treatment wherein the filaments are treated at a temperature not lower than 60 C. but lower than 100 C. and higher than the temperature employed in the step (2), while being subjected to a tension that will maintain the filament length during the treatment at 95-1l5%, preferably 95-110%, of that immediately before the same treatment,

(4) (i) first stage of second heat treatment wherein the filaments are treated at a temperature within a range of l00-l70 C., while being subjected to a tension that will maintain the filament length during the treatment at 95115%, preferably 95-l10%, of that immediately before the same heat treatment, and (ii) second stage of second heat treatment whereby the filaments are further treated at a temperature exceeding 170 C., while being subjected to a tension that will maintain the filament length during the treatment at 95-1 15%, preferably 951l0%, of that immediately before the same heat treatment; the tensions exerted on the filaments in the stages (i) and (ii) of step (4) being so adjusted that the filament length after the stage (ii) of step (4) becomes at least 90% but less than preferably at least 95% but less than 120%, of that immediately before the foregoing step (3).

The polyester filaments having above-described type of load-elongation curve are characterized by the fact that when they are used as tire cord, the cord exhibits markedly improved fatigue resistance and strength-conversion ratio.

. Various test methods have been proposed for measurlng fatigue resistance of tire cord, and results of different test methods do not always correspond. But generally itis safe to conclude that, in case of comparing fatigue resistance of filaments composed of identical polymers, those exhibiting greater toughness (strength x elongation x /2) have higher fatigue resistance. Therefore, in the present specification, toughness will be used as the norm of fatigue resistance. As aforementioned, the fila-.

ments having a load-elongation curve obtained in the tensile test, on which the grade of inclination after the second yield point is very small, have great toughness. Consequently when the yarns from such filaments are twisted together to form tire cord, the cord shows a high fatigue resistance.

Also that the grade of inclination after the second yield point is very small causes the result that the strengthconversion ratio in making the cord fromyarn becomes very high.

Whereas, it is also possible to make polyester filaments showing still different type of load-elongation curve, by another embodiment'of the invention.

That different type of load-elongation curve shows high initial Youngs modulus, andthe maximum grade of in clination after the first yield point much greater than that -'on load-elongation curve of conventional polyester filaments. In an extreme'case of this type of load-elongation curve, the maximum grade after the first yield point on the curve is greater than that between the'origin and first yield point, as illustrated in FIG. 2. Such abnormal mechanical behavior is never seen in conventional polyester filaments. In FIG. 2, the curves (a) and (b) are load-elongation curves of the polyester filaments of this invention and the ordinary polyester filaments, respectively.

The filaments showing such load-elongation curve can be obtained, for example, by performing all the heat I treatments in steps (3) and (4), maintaining the filaments under tension that will render the filament length in each of the treatments 100-130% of that immediately before that treatment, while adjusting the tensions in such a manner that the filaments after the final stage heat treatment of step (4) should have a filament length corresponding to 120150% of that immediately before the heat treatment of step (3).

More specifically, the polyester filaments showing this type of load-elongation curve can be prepared by an embodiment of the invention as described below.

That is, such polyester filaments can be made by a drawing and heat-treating process of polyester undrawn filaments containing at least 90 mol percent of ethylene terephthalate units, which comprises the following steps to be performed in the order stated:

(1) First stage drawing wherein the undrawn filaments are drawn by 1.1-4.0 in a non-solvent liquid of a temperature within a range of 60 100 C.,

(2) Second stage drawing wherein the said undrawn filaments are drawn in a non-solvent liquid of a temperature within a range of 40-75 C. and lower than the temperature employed in the step (1) by at least 10 C., at a draw ratio of 1.2-7.0X and which is at least 80% of the maximum draw ratio,

(3) First heat treatment wherein the so drawn filaments are treated at a temperature within a range of 65 120 C. and which is higher than the temperature employed in step (2), while being subjected to a tension that will maintain the filament length during the heat treatment at 100-130% of that immediately before the same treatment, and

(4) Second heat treatment wherein the filaments are heat-treated, at least once, at temperatures always higher than that employed in step (3), at least one stage heat treatment thereof being performed at a temperature not lower than 120 C., the filaments being subjected in each stage to a tension that will maintain the filament length at 100-130% of that of an immediately preceding stage, and furthermore the tensions exerted on the filaments in the singleor multi-stage heating of step (4) being so adjusted that the length of the filaments after the final stage of step (4) becomes 120-150% of that immediately before the step (3).

The filaments having above-described type of load-elongation curve of course exhibit excellent properties such as high strength and Youngs modulus, and furthermore possess characteristically high mechanical, dimensional stability since their distortion due to elongation under a heavy load is relatively little. Because of such characteristics, these types of filaments are valuable as a material for belt, radial, tire, etc. which are required to have relatively small elongations under heavy loads.

The reason why the polyester filaments characteristically possessing a great number of excellent mechanical properties simultaneously can be obtained by the subject process, is presumably as follows:

8 7 When polyethylene terephthalate. polymers areimparted with fibrous structuref by the unique combinationof dr" and heat-treating steps in accordance witlrthein t ion," the, molecular orientation is hi'ghlyraised bysuch drawing 'jmethod'lwhich 'will prevent formation of cross sctional' anisotropy'as much as possible, and, the crystal;- linity also is suitable adjusted while the cross sectional anisotropy is suitably eliminated. In the conventional, ordinary drawing methods, attempts-to achieve high de gree of orientation invariably'invite structural non-uniformity in the direction of; the cross section-Whereasif cross sectionally uniform structure aimed at, high draw ratios' cannot be "attainedfTh us in practice it has been impossible to make cross sectionally uniform polyf ester filaments which are highly oriented and have high strength. That is, conventional methods" fail to provide such filaments which are uniform in the direction of radius and highly oriented, viz., the filaments having high strength-elongation, toughness and fatigue resistance. Thefailure is presumably caused by the difierent distortions caused by drawing at inside andexte'rior portions of fila ments, and conventional methods cann'otimprove that aspect. The subject process can' successfully eliminate such non-uniformity to quite satisfactory degree, by the novel combination method of drawing and heat-treating steps. i

The filaments prepared in accordance with the inven tion thus exhibit various excellent properties, and are very valuable not only for industrial usages, 'for example, as tire cord, but also for clothing usages. In the latter field, the filaments may be used as they are, or cut into staples. The filaments also command wide utilities in the field of interior decoration. Hereinafter the invention will be explained in further detail with reference to working exampleswhich are given strictly for illustration purpose, but in no way to limit the scope of subject invention. a

EXAMPLE 1 Polyethylene terephthalate undrawn filaments having an intrinsic viscosity of 0.81, a size of approximately 6,100 d./250 fil. and a birefringenceof 0.0015, which had been melt-spun in conventional mannenwere drawn by 2.0x in a -cm. long, warm aqueous bath of 85 C., and further by 3.0x in another -cm. long, warm aqueous bath of 65 C. The filaments were then'heattreated in a 120-cm. long Warm aqueous bathof 85C. while being stretched by 1.05X, followed by the second heat treatment in which the filaments were wound on a.

hot roller of 120 C. and mm. in diameter by two turns and extended by 10%, and then wound on another. hot roller of C. and 150 mm. in diameter, with" stretching by 10%. Then the filaments were taken'up at;

a rate of 100 m./min. The obtained yarn was 860deniers in size, and had a breaking strength of 12.5 g./d'.',-breaking elongation of 15% and Youngs modulus of 2,000 kg./mm. The toughness thereof was 0.94 g./d. The same.

yarn was made into tire cord of 860 .d./2 by imparting thereto right hand under-twisting of 5 6.5 turns: per 10 cm., followed byupper twisting of .two strands together;

culated from the results, which was 81%.

CONTROL 1 Polyethylene terephthalate undrawn filaments having an? intrinsic viscosity of 0.81, a size'of approXimately610 0 d./ 250 fil., and a birefringence of 0.0015, which had been melt-spun in conventional manner, were' drawn and 'heat' treated under the following conditions. Theresults were as given in Table 1. In those experiments, the following standard conditions were adopted in all cases, unless otherwise specified in the same table.

. 9 First stage drawing:

A 100-cm. long aqueous bath of 85 C.

Draw ratio=2.0

Second stage drawing:

First heat treatment: A 120-cm. long aqueous bath of 85 C.

, raised to the range specified in the claims, but the products have inferior properties and fail to provide satisfactory cords.

EXAMPLE 2 Polyethylene terephthalate undrawn filaments having an intrinsic viscosity of 0.92, a size of approximately 7,200 d./ 250 fil. and a birefringence of 0.0028, which had been melt-spun in conventional manner, were drawn by 2.0x in a 100-cm. long, warm aqueous bath of 90 C. and fur- 10 strengthing by 5% ther by 3.0x in a 120-cm. long, warm aqueous bath of 70 C. Then the filaments were sub ected to three suc- Second heat treatment cessive heat treatments as follows, and thereafter wound (i) Temperature 120 (2, up at a rate of 100 m./min.: the filaments were heat- H t ll f 150 'q, treated in a 120-cm. long, warm aqueous bath of 90 C. Stretching by 10% while being stretched by 10%, then wound on a hot roller (ii) Temperature 180 C. of 150 C. and 150 mm. in diameter, to be heat-treated H t ll f 150 mm g under a tension of 10% stretching, and finally wound on Stretching by 10% a hot roller of 200 C. and 150 mm. in diameter while may being stretched by 10%. The obtained yarn was 1,000 1 1 g deniers in size, and had a breaking strength of 13,.5 g./d., Approximately 100 m./m1n. elongation of 16%, and Youngs modulus of 1,900 kg./

TABLE 1 Properties Cord strength (converted Strength Breaking Young's Toughto value per conversion Sample strength Elongation modulus ness 1,000 d./2) ratio No. Conditions of preparation Drawability ('IDB") (g./d.) (percent) (kg/mm?) (g./d.) (kg.) (percent) 1 First stage drawing: temperature= Occasionalmonofilament 0., lower than C. breakage during first and second stage drawings. Drawing inoperable. 2 First stage drawing: temperature= Naps were formed during 110 0., higher than 100 0. second stage drawing.

Drawing operation diificult. 3 First stage drawing: DR"*=1.0, (6.7) 8.8 12 1,250 0.53

lower than 1.1 (DR of second stage drawing 5.0). 4 First stage drawing: DR=4.5, higher Naps were formed during than 4.0 (DR of second stage second stage drawing. drawing=1.2). 5 Second stage drawing: temperature= Naps were formed during 5 0., lower than 10 C. second stage drawing. 6 Second stage drawing: temperature= 7.6) 7.0 16 1,100 0.56

90 0. higher than 85 C. and first stage drawing temperature. First stage drawing: temperature (Ti)XDR=75 C.X2.0. Second 7 stage drawing: temperature (T )X (6.0) 8.8 13 1,300 0.57

DR=80 (1x215, T2 being higher than T 8 Second stage drawing: DR=7.5, Drawing inoperable higher than 7.0. 91 Second stage drawing: DR 80% of (5.2) 7.8 22 1,300

ma.XDR, Le. DR=2.0. 9-2 Conditions are the same as 9-1 except (5.8) 8. 5 14 1,400 0.59

that TDR was increased to 5.8 by rinsing stretch at third and fourth 5 ep. 10 First heat treatment: temperature= Naps were formed during 40 C., lower than C. first heat treatment. 11 First heat treatment: temperature= (7.6) 9.0 10 1,500 0.45

C. lower than second stage drawing temperature. 12 First heat treatment: shrinking by (6.5) 9.1 12 1,350 0.55 12.8

10%, more than 5%. 13 First heat treatment: stretching by Stretching inoperable V 40%. more than 30%. 14 Second heat treatment (i): tempera- Stretching of stage (i) in ture=80 0. lower than first heat second heat treatment treatment temperature. inoperable. 15 Onestage Second heat treatment: 7.0) 9.0 11 1,350 0.49

teg prature=l00 0; lower than 1 16 Second heat treatment (i): shrinking (6.3) 9.2 12 1,280 0.55

by 10%, more than 5%. 17 Second heat treatment (i): stretching Stretching of stage (i) in by 40%,,more than 30%. second heat treatment I a inoperable. 18 Second heat-treatment (ii): shrinking' (6.2) 9 0 14 1,300

by 10%, more than 5%. 19 Second heat treatment (ii): stretching Stretching of stage (ii) in second heat treatment by 40%, more than 30%. v 1

- inoperable.

*-TDR=total draw ratio; DR=draw ratio.

From the results in the table above, it can be understood that when any 'of the drawings and heat treatments is performed under conditions outside the range specified in the claims, either the drawing is hardly operable, or if mm The calculated toughness was 1.08 g./d. This yarn of 1,000 deniers was twisted by 53 turns per 10 cm. as a drawable, TDR is low. In some cases 'IDR may be twisting of 53 turns per 10 cm., to form a tire cord of 1 1 1,000 d./2. The cord strength was 21.3 kg, and elongation was 18%. From those figures the strength-conversion ratio in making the yarn into the cord was calculated, which was 80%. EXAMPLE 3 Polyethylene terephthalate undrawn filaments having an intrinsic viscosity of 0.92, a size of approximately 6,100/d./120 fil., and a birefringence of 0.0020, which had been melt-spun in conventional manner, were drawn by 3.0x in a 100-cm. long warm aqueous bath of 85 C., and further by 2.0x in a l20-crn. long warm aqueous bath of 73 C. Then the filaments were heat-treated in a 120-cm. long, warm aqueous bath of 90 C. under a tension of stretching. As the second heat treatment, the filaments were wound on a hot roller of 150 C. and 150 mm. in diameter under a tension of 8% stretching, and further wound on another hot roller of 195 C. and 150 mm. in diameter under a tension of 10% stretching. whereupon the filaments were withdrawn at a rate of 200 m./min. The resulting yarn had a breaking strength of 12.8 g./d., breaking elongation of 16%, Youngs modulus of 1850 kg./mm. and a toughness of 1.02 g./d.

EXAMPLE 4 From polyethylene terephthalate undrawn filaments having an intrinsic viscosity of 0.80, a size of approximately 6,100 d./250 fil., and a birefringence of 0.0015, which had been melt-spun in conventional manner, a cm. long sample was cut. The sample was drawn by 2.0x at a rate of 10% /min. in an aqueous bath of 55 C. using a frame-type drawing machine, and further by 3.2x at a rate of 50% /min. in an aqueous bath of 45 C. The sample was then heat-treated in 170 C. water while maintaining the constant filament length, further in a 120 C. silicone oil bath under tension of stretching, and finally in a 180 C. silicone oil bath under a tension of 10% stretching. The resulting yarn had a breaking strength of 12.0 g./d., breaking elongation of 18%, Youngs modulus of 2,000 kg./mm.=, and a toughness of 1.08 g./d.

EXAMPLE 5 Polyethylene terephthalate undrawn filaments having an intrinsic viscosity of 0.80, a size of approximately 6,100 d./250 fil., and a birefringence of 0.0015, which had been melt-spun in conventional manner, were drawn by 2.0x in a 100-cm. long warm aqueous bath of 70 C., and further by 3.2x in a 120-cm. long warm aqueous bath of 45 C., at a drawing rate of 40 m./min. Thus drawn filaments were withdrawn after elmination of bath water. The sample was then heat-treated in a l20-cm. long silicone oil bath of 80 C. while being maintained at the constant filament length. As the second heat treatment, the sample was treated in a 100-cm. long silicone oil bath of 120 C. under a tension of 23% stretching, and further in a 100-cm. long silicone oil bath of 185 C. under a tension of 6% stretching. The silicone oil was eliminated from the resulting yarn which was subsequently taken up at a rate of 40 m./min. The yarn had a size of 760 deniers, breaking strength of 11.8 g./d., breaking elongation of 20%, and Youngs modulus of 1900 kg./mm. The calculated toughness of the product was 1.18 g./d.

For comparisons sake, the same undrawn filaments were drawn by 4.3x by means of a hot pin of 90 C. and 60 mm. in diameter, further by 1.3x on a 50-cm. long hot plate of 190 C., and heat-treated on a 50-cm. long plate of 210 C. while being maintained at the contaut filame le h-The arnwas sub equen y ta n.

up at a rate of 60 m./rnin. The resulting yarn had a size of 1,100 deniers, breaking strength of 9.5 g./d., breaking elongation of 13.8%, and a Youngs modulus of 1250 kglmmfi. The calculated toughness of the yarn was 0.65

Thus it is obvious that, the former product exhibits 1-2 markedly higher strength than that of the latter product. The former also has agreater elongation.

EXAMPLE 6 Polyethylene terephthalate undrawn filaments having an intrinsic viscosity of 0.78, a'size of approximately 6,100 d./250 fil., anda birefringence of 0.0016, which had been melt-spun in conventional manner, were cut into the sample length of 10-cm. The sample was drawn by 2.0x in an aqueous bath of6 8 C. at a rate of 25 min., using a frame-type drawing: machine, and further by 3.4x in an aqueous bath of 30 C. at a rate of 100%/ min. The same sample was heat-treated for-,5 minutes in an C. silicone oil bath, under. a tensionof 6 stretching, followed by another heat treatment of 10minutes in a 125 C. silicone oil bath, under a tension of 22% stretching. The resulting yarn had a breakingielon gation of 12.5 g./d., an elongation of 17%, and a Youngs modulus of 2050 kg./mm. The calculated toughness of the product was 1.06 g./d. i

EXAMPLE 7 Polyethylene terephthalate undrawn filaments having an intrinsic viscosity of 0.80, a size of approximately 6,100 d./250 fil., and a birefringence of 0.0015, which had been melt-spun in conventional manner, were drawn by 2.0x in a -cm. long, warm aqueous bath of 93 C., and further drawn by 3.0x in a -cm. long, warm aqueous bath of 65 C. at a drawing rate of 100 m./min.- The filaments were treated to eliminate'thebath water and subsequently heat-treated in a 120-cm.v long, warm: aqueous bath of 93 C., while being stretched" by 10%. Furthermore, the filaments were wound on a hot roller of C. and 150 mm. in diameter by two turns, to be heat-treated under a tension causing 10% stretching, and then wound on a hot roller of 220 C. and 150 mm. in diameter to be heat-treated under atension causing 10% stretching. Thereafter the resulting yarnwas 'withdrawn from the roller at a rate of 100 m./min. The yarn had a size of 860 deniers, a breaking strength of 11.8 g./d., a breaking elongation; of 18%, and a Youngs modulus of 1,800 kg./mm. The calculated toughness was 1.00, g./d. 1

EXAMPLE 8 Polyethylene terephthalate undrawn filaments having an intrinsic viscosity of 0.80, a size of approximately 6,100 d./250 fil., anda birefringence of 0.0015, which. had been melt-spun in conventional manner, were drawn v by 2.0x in a 100-cm."long, warm aqueous bath of 80 C., and further by 3.0x in'a l20-cm.long, warm aqueous bath of 40 C. at a drawing rate of 40 m./min. The filaments were then treated to eliminate the water and wound. Thus drawn filaments were heat-treated in a 100 cm. long silicone oil bath of 80 C. while beingimparted with 5% stretching, then in a 100-cm. longsilicone oil bath of 120 C. under a tension-causing 12 stretching, and finally in a 100-cm. long silicone oil bath of C. under a tension causing 10% stretching ofthe filaments. The resulting yarn was eliminated' of the silicone oil, and wound at a rate of 40 m./mir 1.- The obtained yarn had a breaking strength of 12.8 g./d., breaking "elongation of 14%, and an initial Youngfsmodulus of 1,950 kg./mm. Also the maximum valuelof differential 'coefficient of strength to elongation'after the first yield point was 2,300 kg./mm. The yarns thermal shrinkage at 200 C. was 14.0%, thus exhibiting a very high dimen sional stability. 7

For comparisons sake, the identical undrawn-filaments were drawn by'j4.3 X by means of a hot pin.of 9.0 Grand,

60 mm. in diameter, then further drawn by the'maximum draw ratio of 1.3 x on a 50-cm. long hot-plate of .190.C., and heat-treated on a 50-.cm.; long hot;plateof 210 C., maintaining the constant filament..lengt h.-- The resulting yarn was wound at a rate of .60 m./min.-'1 "he yarn had'a breaking strengthof 9.5 g./ d.,- and a breakingelongationmli 13 "of 13.8%. The initial Youngs modulus was 1250 kg./ mini. The thermal shrinkage at 200 C. was 26.9%.

. EXAMPLE 9 14 ther by 3.0x in a 120-cm. long warm aqueous bath of 65 of 24%. The strength-conversion ratio in making the cord from the yarn was 92%.

CONTROL 2 Polyethylene terephthalate undrawn filaments having an intrinsic viscosity of 0.81, size of approximately 6,100 d./ 250 fil., and a birefrigence of 0.0015, which had been melt-spun in conventional manner, were drawn and heataqueous bath of 70 C. at a rate of 25%/min., using a m frame type drawing machine, and f th r drawn by 3 4 treated under the conditions as specified below. That is, an aqueous bath f C. at a rate f 100% /min T i unless otherwise specified in the following Table 2, all the sample was heat-treated in silicone oil of 80 C. under a eXPefimefltS Were r11H under the below-Specified Standard tension allowing 6% stretching for minutes, then in lonssilicone oil of 125 C. under a tension allowing 20% 15 First Stage drawing: stretching for 5 minutes, and finally in sil cone oil of 180 A 100mm. long aqueous bath of 5 C. ur 1'de r a tension allowing 5% stretching, for 10 IIlll'l- Draw ratio=l0X utes'; The resulting'yarn had a breaking strength of 12.8 I g./d., an elongation of and an initial Youngs Second Stage drawmga modulus of 2,000 kgl/mmP. The maximum differential A g aqueous bath of 65 coefiicient of strength to elongation after the first yield Draw was 2,400 kg./mm. Also the thermal shrinkage of Fi t h t t t t; the yarn silicone oil of 200 C. was 13.8%. A 120-cm. long aqueous bath of 85 C. EXAMPLE 10 Stretching by 3% i 1 20 Second heat treatment:

I Polyethylene terephthalate undrawn filaments having an A 80-cm. long silicone O11 bath of 120 C. intrinsic viscosity of 0.81, a size of approximately 6,100 Stretchmg by 5 0 11/250 fil., and a birefrigence of 0.0015, which had been A 8111mm 011 bath of 200 melt-spun inconventional manner, were drawn by 2.0 X Mamtammg the constant filament lengtha 10'0-cm. long warm aqueous bath of 85 C., and fur- The results are also given in Table 2.

' TABLE 2 Properties Cord strength Grade at (converted Strength 1 Breaking Elon- Tonghfinal part of to value per conversion Sample strength gation ness S-S curve 1,000 d./2) ratio No. Conditions of preparation Drawability (TDR) (g./d.) (percent) (g./d.) (g./d.) (kg) (percent) 1 First stage drawing: DR=1.0, lower (5.4) 8.8 15 10 14.2 31

than 1.1 (DR of second stage drawing=5.0). 2; Second stage drawing: Temp.=90 C., (6.5) 7. 0 16 12 higher than 85 C. and first stage I drawing temp.

{first stage drawing: Temp. (T1)X (5.4) 8.9 16 15 14.0 79 DR=75 o.x2.0. 3 Second stage drawing: Temp. (T1)XDR=80 C.X2.6, T2 being I higher than T1. 4.11.0. One-staged second heat treatment: Naps were formed Temp. (Ti)xDR=s0 o. 1.03, T1 I beinglowerthanfirstheat treatment em 5 Seeon heat treatment (1): shrinking (5.6) 8.6 18 16 13.8 80

by 10%, more than 5%. 6 Second heat treatment (i): stretching Stretching of Stage (i) in.

by 40%, more 30%. second heat treatment inoperable. 7 Second heat treatment (ii): stretching Stretching of stage (ii) in by 40%, more than 30%. second heat treatment inoperable.

C. Then the filaments were subjected to three successive EXAMPLE 11 heattreatments as follows: they were treated in a 120-cm. long, warm aqueous bath of 85 C. under a tension of 3% stretching, then'as'the second heat treatment, in a 80-cm. long silicone oil bath of 120 C., under a tension of 5% stretching, and finally in a 80-cm. long silicone oil bath of 200 C., under a tension maintaining the constant filament length. The filaments were then wound up at a rate of 100 m./ min. The resulting yarn was 1,000 deniers in size, and had a breaking strength of 9.4 g./d., elongation of 2 2%, Young?s modulus of 1,600 kg./mm. and a toughness ofv 1.03 g./d. In the tensile test, the minimum grade of inclination on the load-elongation curve in the part past the second yield point was 4.5 g./d. This yarn of 1,000. deniers was twisted by 53 turns per 10cm. as a right hand under-twisting and two strands thereof were upper twisted together, followed by a left hand twisting of 53 turns per 10-cm., to form a tire cord of 1,000 d./2. The cord had a breaking strength of 18 kg., and an elongation Polyethylene terephthalate undrawn filaments having an intrinsic viscosity of 0.80, a size of approximately 6,100 d./ 250 fil., and a birefringence of 0.0015, which had been melt-spun in conventional manner, were drawn by 2.0 x in a l00-cm. long, warm aqueous bath of 70 C., and further by 2.95 in a l20-crn. long, warm aqueous bath of 45 C., at a rate of 40 m./min. Thefilaments were then treated to eliminate the bath water and wound. The drawn sample was heat-treated in a -crn. long silicone oil bath of 80 C. while being maintained the constant filament length, then in a -cm. long silicone oil bath of C., allowing the sample to shrink by 2%, and finally in a IOO-cm. long silicone oil bath of C. under a tension of 3% stretching. The sample was then eliminated of the silicone oil and wound at a rate of 20 m./

The resulting yarn had a size of 1,050 deniers, strength of 8.4 g./d., and a breaking elongation .of 26%. The

minimum grade of the inclination above the point of 7.0 g./d. on the load-elongation curve obtained in the tensile testl was 4.6 g./d., and the calculated toughness was 1.09 g./

The yarn was then given a right hand under-twisting at a rate of 51 turns per cm., and two strands thereof were upper twisted together, followed by a left hand twisting of 51 turns per 10-cm. Thus a tire cord of 1050 d./2 was prepared, which had a cord strength of 15.9 kg. and an elongation of 28%. From those results, the strength conversion ratio in making the cord from the yarn was calculated to be 90%.

EXAMPLE 12 Polyethylene terephthalate undrawn filaments having an intrinsic viscosity of 0.78, a size of approximately 6,100 d./250 fil., and a birefringence of 0.0016, which had been melt-spun in conventional manner, were cut into a saman aqueous bath of 68 C. at a rate of 10% by 2.0x in an aqueous bath of 68 C. at a rated of 10% /min. using a frame-type drawing machine, and further drawn by 3.2x in an aqueous bath of 40 C. at a rate of 50%/ min. The sample was then heat-treated in silicone oil of 80 C. for 10 minutes, then in silicone oil of 120 C. for 10 minutes, and finally in silicone oil of 185 C. for 10 minutes. Throughout the heat treatments the sample was maintained under tension, maintaining the constant filament length. The sample withdrawn into air was thrown into carbon tetrachloride of room temperature to eliminate the silicone oil and dried. The resulting yarn had a strength of 8.1 g./d. when measured by the method described in the specification, and a breaking elongation of 32%. On the load-elongation curve, the minimum grade of the inclination above the point of 7.0 g./ d. was 4.1 g./d. The calculated toughness was 1.29 g./d. The yarn had an intrinsic viscosity of 0.77 after the above-specified drawing and heating treatments.

EXAMPLE 13 Polyethylene terephthalate undrawn filaments having an intrinsic viscosity of 0.78, a size of approximately 6,100 d./250 fil., and a birefringence of 0.0016, which had been melt-spun in conventional manner, were cut into a sample length of 15 cm. The sample was drawn by 2.0x in an aqueous both of 68 C. at a rate of 10%/ min, using a frame-type drawing machine, and further by 3.2x in an aqueous bath of C. at a rate of 50% min. The sample was heat-treated for 10 minutes in 80 C. silicone oil, for a further 10 minutes in 120 C. silicone oil, and finally for an additional 10 minutes in 185 C. silicone oil. Throughout the heat treatments, the sample was maintained at the constant filament length. The sample was taken out into air, and subsequently thrown into carbon tetrachloride of room temperature to eliminate the silicone oil, and dried. The resulting yarn had a strength of 8.1 g./d. when measured by the test method described in the specification, and a breaking elongation of 32%. On its load-elongation curve, the minimum grade of the inclination after the second yield point was 4.1 g./ d. The yarns calculated toughness was 1.29 g./d

EXAMPLE 14 Polyethylene terephthalate undrawn filaments having an intrinsic viscosity of 0.80, a size of approximately 6,100 d./ 250 fil., and a birefringence of 0.0015, which had been melt-spun in conventional manner, were drawn by 2.0x in a 100-cm. long, warm aqueous bath of 70 C., and further by 2.95 X in a 120-cm. long, warm aqueous bath of C. at a drawing rate of 40 m./min. Thus drawn filaments were treated to eliminate the water and wound.

The sample was then heat-treated in a 100-cm. long silicone oil bath of 80 C. while being maintained at the constant filament length, further in a 120cm. long silicone oil bath of 130 C. with a shrinkage by 2%, and finally in a 100-cm. long silicone oil bath of 190 C. under a tension of 3% stretching. The resulting yarn was treated to eliminate the silicone oil, and wound at a rate of 20 m./min. The yarn had a size of 1,050 deniers, a strength of 8.4 g./d., and a breaking elongation of 26%. On'the load-elongation curve obtained in the yarns tensile test, the minimum grade of the inclination aftenthe, second yield point was 4.6 g./d. Thecalculated toughness-of the yarnwas 1.09 g./d.

For comparisons sake, the similarly drawn samplepf the same filaments was heat-treated in av -cn1.. long silicone oil bath of C. while being allowed. to by 2%, and further in a 100-cm. long silicone oil'bathiof 185 C., under a tension causing 3% stretching. ,The which was treated to eliminate the silicone oil and wound at a rate of 20 rn./min. had a size of 1050 deniers,"a strength of 8.2 g./d., and an elongation ;of. 17%,. '1h" calculated toughness of the product was 0.69 g./d.'0n e load-elongation curve obtained from the tensile testof the yarn, the minimum grade of inclination after thesecond yield point was 12 g./d.. I Y

EXAMPLE 15 1 Polyethylene terephthalate undrawn'filamen ts having an intrinsic viscosity of 0.80, a size of approximately 6,10 0 d./250 fil., and a birefringence of0.0016,'"whi'ch"liadbeen melt-spun in conventional manner, were drawn by 2.0x in warm water of 68 C. at a rate of 10% /min., and further drawn by 3.1 X in warm .water of 40 C, atarate of 50% /min. The drawn filaments. had a ,specificigifavity measured at 20 C. of1;364, birefringenceofj0.'1 85; strength of 7.4 g./d., and a breaking elongationjjof The same filaments were treated inafsiliconefoilfnbatliof 80 C., for 10 minutes,'in another silicone oil'ba'th of 120 C., for 10 minutes, and in still another silicone oil bath of 185 C. for additional 10 minutesJIhroughoutthe heat treatments the filaments were subjected to the tension maintaining the constant filament length. The filaments subsequently withdrawn into air were washed to remove the silicone oil pick-up with carbon tetrachloride. The product had a breaking strength of 8.0. g. /.d., ahd li'n elongation of 39%. 7 3 1 r 1 'For comparisons sake, similarly drawn filaments were treated in a silicone oil bath of 185 C. for 10 minutes. while being maintained at the constantfilament length; withdrawn into air, and treated to eliminatezthe silicone oil with carbon tetrachloride. The resulting filaments had a breaking strength of 7.2 g. /d., and an: elongation of 37%. I. a i.

The calculated toughness of. the above two productS were, respectively, 1.56 g./d. for the, former and 1.32 g./ d. for the latter. Thus the former exhibited a far greater toughness. EXAMPLE 16 Polyethylene terephthalate undrawn filaments having an intrinsic viscosity of 0.80, .a size of approximately 6,100 d./250 fil., and a birefrin QnCe of 0,0015, whicl 1 had been melt-spun in conventional manner, were drawn by 1.4 in a 100-cm. long, warm aqueous bathfo'f 905' C., and further drawn by 4.3x in a 120-cm.jflo r 1'g ,iwa aqueous bath of 65 0. Then the filaments were'lh treated in a LOO-cm. long, warm aqueous bath of "85" C. retaining the constant filament length, ensign wound on a hot rollerof 160 C. and in diarnfeter by three turns and allowed to stfiet'ch'by 3%,, the filaments were contacted witha' 500-mm. long' hot plate of 230 C. under a tension casuihg3%, strtfhin' and wound atarate of 120m./min. j

The resulting yarn had a size of 1,000'fidenier's; a strength of 8.4 g./d., and a. breakingelongation of j20%} On the load-elongation curve obtainediiithe tensiletest of the yarn, the minimum grade of'the inclination aftij the second yield point was 4.8 g./d. The calculated toiighf ness was 0.84 g./d.

Then the yarn of 1,000 denierswasright hand under: twisted by 53 turns per 10 cm., and two strandsther'e of were together given an upper twisting followed byaleft 17 hand twisting of 53 turns per -cm. Thus a tire cord of 1,000 d./2 was prepared. The cord had a strength of 15.4 kg. and an elongation of 24%, the strength-conversion ratio in making the cord from the yarn, which was calculated from the above results, being 92%.

EXAMPLE 17 Polyethylene terephthalate undrawn monofilament of 1,200 deniers having an intrinsic viscosity of 0.80 and a birefringence of 0.0035, which had been melt-spun in conventional manner, was drawn by 2.0x in warm water of 65 C. at a rate of 10%/min., using a frame-type drawing machine. The filament was further drawn by 3.1x in warm water of 45 C., at a rate of 100% /min., and thereafter heat-treated in warm water of 65 C., with stretching by 1.1x. As the second heat treatment, the filament was further treated in silicone oil of 120 C. with stretching, and then in silicone oil of 180 C. with 10% stretching. The resulting monofilament had a breaking strength of 13.8 g./d., breaking elongation of 16%, and a Youngs modulus of 1,900 kg./mm.

CONTROL 3 Polyethylene terephthalate undrawn filaments having an intrinsic viscosity of 0.90, a size of approximately 5,800 d./250 fil., and a birefringence of 0.0020, which had been melt-spun in conventional manner, were drawn by 4.3x as wound on a hot pin of 92 C. in the conventional manner, and further drawn by 1.3X on a 50-cm. long hot plate of 190 C. Subsequently the filaments were heat-treated on a 50-cm. long hot plate of 210 C. while being allowed to shrink by 7%, and wound at a rate of 80 m./min. The resulting yarn had a strength of 7.5 g./d., and an elongation of 15 On the load-elongation curve, the minimum grade of inclination above the point of 7.0 g./d. was g./d. The calculated toughness was 0.56 g./d. The yarn size was 1,100 deniers, and intrinsic viscosity was 0.88. The same yarn was given a right hand under-twisting of 51 turns per 10 cm., and two strands thereof were together given an upper twist, followed by a left hand twisting of 51 turns per 10-cm. Thus a tire cord of 1,100 d./2 was obtained. The cord had a strength of 14.4 kg, and an elongation of 17%. The strengthconversion ratio in making the cord from the yarn was 82%.

What is claimed is:

1. A drawing and heat-treating process of polyester undrawn filaments containing at least 85 mol percent of ethylene tcrephthalate units, which consists essentially of:

'(1) drawing said undrawn filaments in a first stage drawing by 1.14.0 at a temperature within a range of 40-100 C.,

(2) drawing said filaments drawn in step (1) in a second stage drawing at a temperature within a range of 10-85 C., said temperature being lower than that employed in step (1), at a draw ratio within a range of 1.2-7.0X, said draw ratio being at least 80% of the highest possible draw ratio under the conditions of drawing without filament breakage,

(3) heat-treating the drawn filaments of step (2) in a first heat treatment at a temperature within a range of 55 -120 C. and higher than that employed in step (2), while subjecting said filaments to a tension that maintains the filament length during said first heat treatment at 95-130% of that immediately before said first heat treatment,

(4) further heat-treating said filaments in a second heat treatment conducted in at least one stage, at a temperature higher than that employed in step (3), at least one stage of said second heat treatment being performed at a temperature not lower than 110 C., said filaments being subjected in each stage to a tension that maintains the filament length at 95- 130% of that of an immediately preceding stage, the tension exerted on the filaments in each stage of step (4) being so adjusted that the filament length after the final stage of step (4) is at least of that immediately before step (3).

'2. The process described in claim 1, wherein the drawing temperature of step (2) is lower than that employed in step (1) by at least 10 C.

3. The process described in claim 1, wherein the heattreating temperature of step (3) is higher than the drawing temperature of step (2) by at least 10 C., and the heat-treating temperatures in all stages of step (4) are higher than that employed in step (3) by at least 10 C.

4. The process of claim 1, wherein the drawings in steps (1) and (2) are performed in water or ethylene glycol.

5. The process of claim 1, wherein the filaments after the step (3) are further heat-treated at least once in step (4) at a temperature not lower than 120 C.

6. The process of claim 1, wherein the stretch ratios in steps (1) to (4) are so adjusted that the filaments after step (4) have a length 6 to 10 times that of the undrawn filaments.

7. The process of claim 1, wherein the tension exerted on the filaments in each stage of step (4) is so adjusted that the filament length after the final stage of step (4) is at least 90% but less than 120%, of that immediately before step (3).

8. The process of claim 1, wherein the tension exerted on the filaments in each stage of step (4) is so ad justed that the filament length after the final stage of step (4) is 120l70%, of that immediately before step (3).

9. The process of claim 7, which consist essentially of:

(1) drawing said undrawn filaments in a first stage drawing by 1.14.0 in water or ethylene glycol of a temperature within a range of 40-100 C.,

(2) drawing said filaments drawn in step (1) in a a second stage drawing in water or ethylene glycol of l a temperature within a range of 10-85 C., said temperature being lower than the temperature employed in step (1) at a draw ratio of 1.27.0 said draw ratio being at least 80% of the highest possible draw ratio under the conditions of drawing without filament breakage,

(3) heat-treating the drawn filaments of step (2) in a first heat treatment at a temperature not lower than 60 C. but lower than 100 C. and higher than the temperature employed in step (2), while subjecting the filaments to a tension that maintains the length of said filaments during said first heat treatment at 115% of that immediately before said first heat treatment,

(4) further heat-treating the filaments from step (3) in a two-stage second heat treatment comprising a (i) first stage of second heat treatment at a temperature within a range of -170 C., while the filaments are subjected to a tension that maintains the length of said filaments during the first stage of second heat treatment at 95- of that immediately before the first stage of second heat treatment, and (ii) a second stage of second heat treatment at a temperature exceeding 170 C., while subjecting the filaments to a tension that maintains the length of said filaments during the second stage of second heat treatment at 95-115% of that immediately before the second stage of second heat treatment, the tension exerted on the filaments in stages (i) and -(ii) of step (4) being so adjusted that the filament length after stage (ii) of step (4) is at least 90% but less than of that immediatelyl before step (3).

10.The process of claim 8, which consists essentially (1) drawing said undrawn filaments in a first stage drawing by 1.1-4.0X in water or ethylene glycol of a temperature within a range of 60-100 C.,

(2) drawing said filaments drawn in step (1) in a second stage drawing in water or ethylene glycol of a temperature within a range of 4 -75 C. and lower than that employed in step (1) by at least 1 0 C., at a draw ratio of 1.27.0 and which is at least C., at a draw ratio of 1.27.0 and which is at least of the highest possible draw into under the conditions of drawing without filament breakage,

(3) heat-treating the drawn filaments of step (2) in a first heat treatment at a temperature within a range of 65 1'20 C., said temperature 'being higher than that employed in step (2), while subjecting the filaments to a tension that maintains the filament length during the first heat treatment at 130% of that 20 immediately before the first heat treatment, and (4) further heat-treating the filaments of step -(3) in a second heat treatment conducted in at least one stage at temperatures higher than that employed in 20 step (3), at least one stage thereof being performed at a temperature not lower than C., the filaments being subjected in each stage to a tension that maintains the filament length at IOU-% of that of an immediately preceding stage, the tension exerted on the filaments in each stage of step (4) being so adjusted that the filament length after the final stage of step (4) is 120150% of that immediately before step (3 References Cited UNITED STATES PATENTS JULIUS FROME, Primary Examiner H. MINTZ, Assistant Examiner 264-290 T, 342 RE US. Cl. X.R. 

